The present invention relates to ascertaining the surface level of a metal bath in a metallurgical vessel, for example, of the type used for steel making e.g. a converter.
The steel bath in a metallurgical vessel is usually covered with a layer of slag the layer being possibly of variable thickness. In cases it is necessary to ascertain the precise level of the liquid steel i.e. of the steel-slag-interface. In order to ascertain that interface or liquid bath level, it is known to utilize the difference in electric conductivity of steel and slag; the electric conductivity exhibits a pronounced discontinuity at that level and interface. For this purpose it has been suggested to use a probe which is comprised of two electrically insulated electrodes i.e. they are insulated from each other and are made of a material which is resistive to temperature to considerable extent. The electrodes, moreover, are covered and protected by a steel cap in order to prevent slag depositing on the electrodes proper as they are inserted into the bath. A constant DC voltge is applied to these electrodes by means of a flexible cable.
Earlier, the probe has been placed on a lance or rod and is now lowered into the metal bath by means of an appropriate transport device. The cap will melt in the steel and a DC current is now flowing between the electrodes, both being immersed in the bath and they are electrically interconnected by means of the liquid steel. The electric current flows in addition through a particular electrical resistance and a particular voltage drop can be taken from across that resistance, the voltage drop being indicative of the current that flows between the two electrodes.
As the lance is moved up the electrodes will also traverse the slag layer and the interface between slag and steel. As the electrodes traverse the interface the electric conditions and particularly the resistance of the current path between the two electrode changes drastically. That change reflects in a change in the voltage drop across the measuring resistor, and the voltage drop can be used to actuate an electronic threshold switch. The threshold switch includes a comparator whose response level is adjustable by means of applying a variable and adjustable reference voltage to one input of the comparator, the other input receives the measuring voltage. As soon as the threshold so adjusted is traversed, an indication is provided. The transport device lowering and lifting the lance may be constructed so that a running indication is provided as to the vertical level change of the two electrodes. The response of the comparator as described is then referenced against that particular indication which is deemed to indicate the level of the steel bath and its surface.
It was found, however, that the method explained above, may experience a variety of interferences modifying the current through the electrodes and, therefore, the voltage across the measuring resistor and these variations may in effect shift the particular response of the comparator which means that the measuring result is subject to an unforeseeable error. For reasons of interference in the external circuitry a two conductor system is used and the current through the steel is limited by the resistance of the cable and by the measuring resistance. Thus, the measuring result depends to some extent upon the length of the cable; in practice, the cable leading to the measuring electrodes maybe 50 meters long or more. Occasionally, in particular in the case of repairs, the length may be changed, also the transition resistances in the circuit including the transition resistance to and from the electrodes influence the current further. Finally, cable and electrodes may vary in temperature and that in turn may also result in a change in current. It can thus be seen that the known method now seemingly advantageous in principal, is prone to a number of interferences resulting in erroneous readings.